The invention relates to an electrochromic element comprising a substrate and an electrochromic layer having a thickness d on the basis of a metal oxide selected from the group consisting of tungsten oxide, molybdenum oxide, niobium oxide, manganese oxide and zirconium oxide or combinations thereof, the transmission properties of the electrochromic layer in the visible range being governed by an applied voltage difference across the electrochromic layer.
The invention also relates to a display device provided with a display screen and an electrochromic element.
The invention further relates to a display device comprising a plurality of pixels including an electrochromic element.
The invention further relates to a method of manufacturing an electrochromic layer on the basis of a metal oxide selected from the group composed of tungsten oxide, molybdenum oxide, niobium oxide, manganese oxide and zirconium oxide or combinations thereof, the transmission properties of the electrochromic layer in the visible range being governed by an applied voltage difference across the electrochromic layer, said electrochromic layer being provided by means of a sputter-deposition process in an evacuated chamber.
Electrochromic elements for varying the transmission of light are used to influence the transmission and/or reflection of (visible) light, for example, of lamps, of rear view mirrors and of sunroofs for cars, or of windows for buildings (smart windows), or of lenses, or of spectacle lenses for (sun) glasses. They are also used on the viewing side of display windows of (flat-panel) display devices, such as cathode ray tubes (CRTs), plasma display panels (PDPs) and liquid-crystal display devices (LCDs, LC-TVs and plasma-addressed LCDs) to improve the contrast of the image reproduced. By virtue thereof, the feasibility of readily bringing the light transmission to a desired value is increased and, for example, the necessity of changing the glass composition of the display window of a display device is avoided.
The electrochromic elements mentioned in the opening paragraph influence the intensity of reflected ambient light and the intensity of light originating from an (internal) light source, such as the phosphors in a CRT. Incident ambient light passes through the electrochromic element and is reflected at the substrate or at the phosphors of color filters in a CRT, whereafter the reflected light passes through the electrochromic element again. If the transmission of the electrochromic element amounts to T, the intensity of the reflected ambient light decreases by a factor of T.sup.2. Light originating from the internal light source(s), however, passes through the electrochromic element only once, so that the intensity of this light decreases only by a factor of T. The combination of these effects leads to an increase of the contrast by a factor of T.sup.-1.
Oxides of specific transition metals are capable of accepting guest atoms such as hydrogen and alkali-metal atoms. If the oxide forms part of an electrochemical cell (the electrochromic element), the guest atoms can be accepted and released again in a reversible manner. In general, an electrochromic element comprises a first (transparent, conductive) electrode which is connected to an electrochromic layer, the so-called work electrode, and a second (transparent, conductive) electrode, the so-called counter electrode, which contains a material which serves as a source and as an acceptor for the guest atoms, and an ion-conducting (liquid, polymeric or solid) material, the so-called electrolyte, being present between said two electrodes. The transmission properties of the electrochromic element in the visible range undergo a change when a voltage difference is applied across the electrochromic element. As the variation in light is often detected via a light sensor provided in the vicinity of the electrochemical cell, this is referred to as an indirect response to a variation in light.
An electrochromic element of the type mentioned in the opening paragraph disclosed in United States patent U.S. Pat. No. 4,824,222. In said patent, a description is given of an electrochromic element which comprises an electrochromic layer containing molybdenum oxide (MoO.sub.3), tungsten oxide (WO.sub.3), zirconium oxide (ZrO.sub.2) or niobium oxide (Nb2O.sub.5), or combinations thereof, as the electrochromic metal oxide.
Such electrochromic elements often have the disadvantage that their transmission spectrum is not color neutral when the voltage difference is applied across the electrochromic layer (the so-called "colored" state).